Evidence for lunar volcano-tectonic features

Abstract

Objections to large-scale lunar volcanism are partly based on the small maximum diameter of terrestrial volcanic craters compared with lunar craters and on the association of terrestrial volcanoes with oceanic ridges and geosynclinal-orogenic belts, which seem to be absent on the moon. Actually, terrestrial volcano-tectonic complexes comparable in size (diameter up to 180 km) and morphology, to large lunar craters are also distributed independently of oceanic ridges and geosynclinal-orogenic belts; their relationships to tectonic plates are poorly understood. They are characteristic of areas of fracture tectonics, including platforms, and outnumber known impact craters between 5 and 10:1. Volcanic rocks that erupted from volcano-tectonic complexes have episodically ‘overplated’ continents, contributing significantly to their crust. On the moon the volcanic episodes appear to have been fewer, and endogenic forces were insufficient to initiate geosynclines, welts resembling oceanic ridges, and tectonic plates. Instead, ‘overplating’ may have played a major role in forming the lunar surface during devolatilization early in lunar history. If so, it resulted in formation of volcano-tectonic ‘craters’ controlled by the lunar tectonic grid, a system of fractures similar to those of the earth's platforms. Terrestrial volcano-tectonic complexes that characterize overplated areas tend to be rimmed by volcanic and subvolcanic domes, arcuate-to-circular or polygonal in plan (confocal subvolcanic or 'ring-dike complexes). A sample of over 400 reveals at least four major types: (1) calc-alkalic to peralkalic parent magma, (2) high-alkali feldspathoidal complexes, (3) basaltic complexes, and (4) layered complexes of ultramafic rocks and anorthosite.